Myths and Legends of our Own Land, by Charles M. Skinner, [1896], at sacred-texts.com
Some Indians believe that the Thunder Bird is the agent of storm; that the flashes of his eyes cause lightning and the flapping of his cloud-vast wings make thunder. Not so the Passamaquoddies, for they hold that Katahdin's spirit children are Thunders, and in this way an Indian found them: He had been seeking game along the Penobscot and for weeks had not met one of his fellow creatures. On a winter day he came on the print of a pair of snow-shoes; next morning the tracks appeared in another part of the forest, and so for many days he found them.
After a time it occurred to him to see where these tracks went to, and he followed them until they merged with others in a travelled road, ending at a precipice on the side of Katahdin (Great Mountain).
While lost in wonder that so many tracks should lead nowhere, he was roused by a footfall, and a maiden stepped from the precipice to the ledge beside him. Though he said nothing, being in awe of her stateliness and beauty, she replied in kind words to every unspoken thought and bade him go with her. He approached the rock with fear, but at a touch from the woman it became as mist, and they entered it together.
Presently they were in a great cave in the heart of Katahdin, where sat the spirit of the mountain, who welcomed them and asked the girl if her brothers had come. "I hear them coming," she replied. A blinding flash, a roar of thunder, and there stepped into the cave two men of giant size and gravely beautiful faces, hardened at the cheeks and brows to stone. "These," said the girl to the hunter, "are my brothers, the Thunder and the Lightning. My father sends them forth whenever there is wrong to redress, that those who love us may not be smitten. When you hear Thunder, know that they are shooting at our enemies."
At the end of that day the hunter returned to his home, and behold, he had been gone seven years. Another legend says that the stone-faced sons of the mountain adopted him, and that for seven years he was a roaming Thunder, but at the end of that time while a storm was raging he was allowed to fall, unharmed, into his own village.